1. Field of the Invention
The present invention relates to a power transfer device adapted for use in combination with a power transmission for a four-wheel drive vehicle to selectively transfer output drive torque from the power transmission to a pair of front-wheel axles and/or a pair of rear-wheel axles, and more particularly to a power transfer device capable of selectively providing either a low speed four-wheel drive, a high speed four-wheel drive or a high speed two-wheel drive.
2. Discussion of the Background
In Japanese Utility Model Early Publication No. 53-121642, there has been proposed a power transfer device of this kind which comprises a first shaft rotatably mounted within a housing and having an input end for connection to an output shaft of a power transmission and an output end for drive connection to a pair of rear-wheel or front-wheel axles, a second shaft rotatably mounted within the housing in parallel with the first shaft for drive connection to a pair of front-wheel or rear-wheel axles, a drive gear mounted on the first shaft, and a driven gear mounted on the second shaft and drivingly connected to the drive gear. In such a conventional power transfer device, the drive gear is rotatably mounted on the first shaft and connectable to the first shaft by means of a changeover mechanism such as a clutch in the transfer device, while the driven gear is fixedly mounted on the second shaft for rotation therewith. Alternatively, the drive gear is fixedly mounted on the first shaft for rotation therewith, while the driven gear is rotatably mounted on the second shaft and connectable to the second shaft by means of a changeover mechanism such as a clutch in the transfer device.
To select two-wheel drive at the power transfer device, the changeover mechanism is conditioned to disconnect the drive gear from the first shaft or the driven gear from the second shaft. During travel of the vehicle in such a two-wheel drive mode, the non-driven wheels act to drive the drive line components between the non-driven wheels and the changeover mechanism in the transfer device. This causes various problems such as temperature-rise of lubricating oil in the transfer device, wear of the components, power consumption, unpleasant gear noises and the like. Although such problems can be solved by the use of manual or automatic hub locks at each non-driven wheel to disconnect the wheel from its associated split axle part when the two-wheel drive is selected at the transfer device, the hub locks are either inconvenient to operate or expensive and complicated. Additionally, if the driver forgets to unlock the manual hub locks in the two-wheel drive mode of the vehicle, the forgoing problems may not be avoided.